This invention pertains to video signal encoding/decoding systems, but more specifically, to a method and apparatus for restoring to proper video format, a video signal which has previously been encoded with anti-copy protection signals. The invention finds its widest use in restoring video signals which are prerecorded on a video recording medium, such as signals recorded on a videotape of a video cassette.
By its U.S. Pat. No. 4,631,603 and by its copending patent application U.S. Ser. No. 935,055, the assignee hereof disclosed anti-copy protection systems for use with video signals. The system disclosed in the '603 patent is finding significant usage with video signals on recorded video cassette tapes. The addition of the copy-protect signal(s) to a prerecorded video cassette tape can interfere with unauthorized copying of the tape but still permit viewing of the program from the original tape. The anti-copy protection systems in widest use at the time this patent application is filed include the addition to the normal video signal, of unique ordered pairs of pseudo-synchronizing pulses and positive pulses placed at certain locations in the video signal so as to have the effect of inhibiting proper AGC (automatic gain control) adjustment in a video tape recorder (VCR) during recording. As a result, the picture quality obtained from a copy of the video signal having the copy-protect signals is significantly deteriorated. However, these unique copy-protect signals do not affect normal viewing of the video material using a conventional television set.
To explain the copy-protect processes, a typical video signal defines normal viewing fields (e.g., those parts defining the display for a television set or monitor), horizontal blanking intervals, and vertical blanking intervals between normal viewing fields. Both the viewing fields and the vertical blanking intervals comprise series of horizontal scan lines carrying picture data and control information that include for each line in an associated horizontal blanking interval, a horizontal synchronizing (or sync) pulse. The horizontal sync pulses are used for horizontally registering successive scan lines. Other synchronizing pulses, e.g., broad pulses and equalizing pulses, normally occur during the vertical blanking interval. These pulses generally are called vertical synchronizing ("sync") pulses because they occur only during the vertical blanking interval. A television monitor or set, and also the processing circuitry associated with VCR copying, use the vertical sync pulses for vertical synchronization.
It should be noted that there are portions of a normal viewing field defined by a video signal that typically are not used for the actual display. More specifically, the horizontal scan lines immediately adjacent to those portions of a video signal defining vertical blanking intervals, are not displayed by typical television monitors or sets. Because of this, those signal portions immediately adjacent to vertical blanking intervals are also available in their entirety for the addition of copy-protect signals. Thus, insofar as this invention is concerned, such signal portions may also be considered part of the vertical blanking intervals, and the term "vertical blanking interval" as used herein is meant to encompass the same unless it is clear from the usage that only the actual vertical blanking interval is meant.
The aforementioned copy-protection systems of assignee alter a normal video signal by adding positive (AGC) and/or pseudo-sync pulses after at least some of the native sync pulses. By "added" pulses are meant pulses which are formed in the video signal to prevent copying, i.e., signals which supplement the normal video signal carrying typical picture data and control information. That is, the term "added" as used herein refers to the type of signal, rather than to the time at which it may be incorporated with the remainder of the video signal. In the arrangement disclosed in U.S. Pat. No. 4,631,603, the added pulses appear in the vertical blanking interval and interfere with a VCR's recording AGC circuitry.
Under certain circumstances, there is a need to disable the anti-copy protection system to permit recording of copy-protected recordings, such as, for example, during authorized and permitted copying or for studio editing purposes. Assignee's U.S. Pat. No. 4,695,901 discloses several embodiments of one such system. It is an objective of the present invention to provide an improvement for disabling an anti-copy protect system.